A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
One example of an ion source widely used in MS is an electron ionization (EI) source. In a typical EI source, sample material is introduced into an ionization chamber in the form of a molecular vapor. An electron emitter, typically a thermionic cathode such as a heated filament composed of a refractory material (e.g., tungsten), is employed to emit energetic electrons. The emitted electrons are then collimated and accelerated as a beam into the ionization chamber under the influence of a potential difference impressed between the filament and an anode. The sample material is introduced into the ionization chamber along a path that intersects the path of the electron beam. Ionization of the sample material occurs as a result of the electron beam bombarding the sample material in the region where the sample and electron paths intersect. The primary reaction of the ionization process may be described by the following relation: M+e−→M*++2e−, where M designates an analyte molecule, e− designates an electron, and M*+ designates the resulting molecular ion. That is, electrons approach a molecule closely enough to cause the molecule to lose an electron by electrostatic repulsion and, consequently, a singly-charged positive ion is formed. A potential difference is employed to attract the ions formed in the ionization chamber toward an exit aperture, after which the resulting ion beam is accelerated into a downstream device such as the mass analyzer or first to an intervening component such as an ion guide, mass filter, etc.
The electric field utilized to accelerate the electrons into the ionization chamber is usually generated by a filament voltage that is negative (or less positive) relative to the ionization chamber voltage. In many EI ion sources, a more negative electron repeller, positioned further away from ionization chamber, is used to push more electrons to enter the ionization chamber. In some of the known EI ion sources, an electron lens is disposed between filament and ionization chamber to pull electrons away from the filament. While electrons collide with gas samples, sample neutrals are ionized if the electron energy is larger than sample ionization potentials. Commonly, the electron beam enters the ionization chamber with an energy of around 20-150 eV since the typical sample ionization potential is between 7.5 to 15 eV. In such an EI ion source, molecules are extensively fragmented and library-searchable mass spectra are accomplished. However, in some cases, for example in cases involving structure elucidation or identification of unknown compounds, mass spectra with rich molecular ions and/or higher mass diagnostic ions are preferred. This has been practiced in some of the known EI ion sources by operating at a lower electron energy (8-20 eV), which is called “low electron energy EI” or “soft EI.” In the soft EI mode, the voltage difference between the filament and the ionization chamber needs to be set at near the sample ionization potential, e.g., 10 eV, which results in low electric field strength between the filament and the ionization chamber. Unfortunately, the low electric field strength prevents the EI source from generating a stable higher intensity electron beam. Thus, past attempts to implement soft ionization via EI have been limited to producing undesirably low EI signal intensity.
Generally, when electron energy is above 20 eV, known EI ion sources show reasonable performance. However, when electron energy is less than 20 eV, it is difficult for known EI ion sources to generate a stable and high intensity low electron energy electron beam. Thus, known EI ion sources are not optimized for soft EI.
Therefore, there is a need for EI ion sources that are more effective for implementing soft ionization.